Atomic absorption spectrometers serve to determine the amount or concentration of an element looked for in a sample. For this purpose a measuring light beam from a line-emitting light source, a hollow cathode lamp, for example, is directed to a photo-electrical detector. An atomizing device is arranged in the path of rays of this measuring light beam. The sample which is to be analyzed is atomized in this atomizing device such that the components of the sample are present in an atomic state. The measuring light beam contains the resonant lines of the element looked for. These resonant lines of the measuring light beam are absorbed by the atoms of the element looked for in the cloud of atoms, while, ideally, the other element contained in the sample do not influence the measuring light beam. Therefore, the measuring light beam is subjected to an attenuation, which is a measure of the number of the atoms looked for that are in the path of the measuring light beam. Thus, a measure of the concentration or the amount of the looked for element in the sample is obtained depending on the method of atomization applied.
For highly sensitive measurements electrothermal atomization is preferably used. In this method the sample is introduced into a furnace which is heated to a high temperature by passing electrical current therethrough. Thereby, the sample is dried, ashed and then atomized. Then a "cloud of atoms" is generated within the furnace in which cloud the atom looked for is present in an atomic state. The measuring light beam is passed through this cloud in the furnace.
Tubular furnaces for the electrothermal atomization of samples for atomic absorption spectroscope are known as "graphite tube atomizers". These furnaces consist of a small tube made of graphite which is held between two annular contacts. A high electrical current is passed through the contacts through the tube in its longitudinal direction. Thereby, the tube can be heated to high temperatures. The sample is inserted into the tube through a lateral inlet port and is atomzied when the tube is heated. The measuring light beam passes through the annular contacts and the bore of the tube in its longitudinal direction. The graphite tube is surrounded both inside and outside by an inert gas which prevents the tube from coming into contact with air or oxygen. Such graphite tube atomizers are illustrated and described in German patent application 23 14 207 and German patent application 21 48 783, which corresponds to U.S. Pat. No. 4,098,554 issued Jul. 4, 1978 and U.S. Pat. No. 3,778,156 issued Dec. 11, 1973, respectively.
From German patent application 25 54 950 which corresponds to U.S. Pat. No. 4,111,563 issued Sep. 5, 1978, a furnace for electrothermal atomization of liquid samples (graphite tube) is known, in which a tubular inner body is provided in the interior of the actual tubular furnace body. The inner body is arranged concentrically in the furnace body and extends only through the central area of the furnace body. In its center the furnace body is provided with a lateral inlet port. An inlet port aligned therewith is arranged in the tubular inner body. The inner body is connected to the furnace body through longitudinally extending webs, which extend in the longitudinal plane perpendicular to the inlet port.
This design of the furnace prevents disturbances which are caused by the inserted liquid spreading over large areas of the inner wall of the graphite tube and thus reaching relatively cool end portions of the tubular furnace. At the end portions, only incomplete vaporization is effected such that sample material is retained which disturbs subsequent measurements of other samples. Also, sample losses are avoided which can occur by the seeping of sample liquid into the porous graphite.
It is desirable to delay the atomization of the sample relative to the heating of the wall of the furnace. This is to ensure that the components of the sample, when they are atomized, do not precipitate onto relatively cool wall portions and that the sample is atomized as abruptly as possible. This provides a strong absorption signal to occur. From L'vov's publication in "Spectrochimica Acta" vol. 33B, 153-193, a platform, having a generally rectangular shape, made of pyrolytic graphite is known which is inserted into a furnace designed as a graphite tube. In order to reduce the contact with the graphite tube, wall grooves are provided along the longitudinal edge of the platform. In this way the sample is heated mostly indirectly by radiation of the inner wall of the furnace.
German patent application 29 24 123 which corresponds to U.S. Pat. No. 4,303,339 issued Dec. 1, 1981 shows a tubular furnace body (graphite tube) with a platform which is provided with a recess for accommodating the sample and is guided into the furnace body only along two opposite longitudinal edges.
The amount of sample which can be accommodated by this platform is limited. Furthermore, for the user the problem arises in that into a small furnace body an even smaller platform has to be inserted. This is a very complicated manipulation. Part of the electrical current flowing through the furnace body in its longitudinal direction also flows through the platform. Therefore, the platform is not only heated indirectly by radiation, but Joul's heat is generated within the platform itself.
From German patent application P 37 43 286, not pre-published, which corresponds to U.S. patent application No. 285,884 filed Dec. 16, 1988, it is known to arrange a hollow, generally semicylindrical inner body which is connected to the outer furnace body by a web, in a tubular furnace body. In this way the hollow, generally semicylindrical inner body is made integral with the tubular furnace body such that problems of handling are omitted. The inserted sample "faces" the wall of the outer furnace body, that is, it is indirectly heated by the radiation of this wall.
In this furnace a web is arranged symmetrically to the longitudinal center plane of the inner body and in the center of the inner body. However, the sample which is to be atomzied is dosed in this inner body center. Since the furnace is naturally arranged such that the hollow, generally semicylindrical inner body as "platform" extends substantially horizontally, i.e., the longitudinal center plane of the inner body extends vertically, the dosed sample accumulates in the area of the longitudinal center plane at the lowest point of the inner body. Thus, the dosed sample is located directly above the web. The unavoidable heat supplied by heat conduction through the web is exactly at the location where the sample which is to be atomized is located. This counteracts the desired heating of the sample with a delay relative to the heating of the outer furnace body.